Water-activatable adhesives are among the most common type of adhesives generally used by the general public. These include such uses as envelope-flap adhesives, postage stamp adhesives, bindery tapes and sealing tapes. In all such uses the dry adhesive in situ, on a substrate, usually of paper film or cloth, is activated by moisture, water, or commonly saliva, to provide an active adhesive for adhering the substrate to the desired surface.
Water-activatable adhesives can be applied as a coating to the substrate from solution or emulsion in either water or organic solvent and then drying the coating by evaporation or distillation of the water or solvent. The coating can also be applied by coating from a hot-melt of the adhesive. The coated article is then ready for storage until ready for use, at which time the coating is moistened and thus activated.
Water-activatable adhesive coats, applied from organic solvent solutions, entail complex and costly procedures since the solvent must be confined and collected either for cost recovery, or environmental considerations.
Similarly, coats prepared from aqueous solutions or waterbased emulsions also entail considerable energy expenditure in rapidly heating the coated matrix to remove the water as solvent and thus dry the adhesive coat sufficiently for storage until used.
Hot-melt adhesives entail none of the above complexities but do have certain problems. The hot-melt adhesives are generally applied by melting the composition and then coating a molten layer on the substrate. This coated material is then cooled to harden the coat and is ready for storage.
Hot-melt adhesive compositions must meet certain stringent criteria. The melting point of the composition must be sufficiently low so that the molten mass need not be heated to temperatures which would affect the substrate or would cause decomposition of the adhesive in the applicator machinery (pot-life).
The melted composition should have a suitably low viscosity for use in applicators and a sufficiently low liquid surface-tension or surfactancy to properly wet the substrate with the adhesive. The coating composition must be sufficiently flexible so that the hardened coat will not flake off.
The hot-melt coating must also be capable of storage under conditions normally encountered without blocking, i.e. adhering, due to activating of the adhesive by the combination of ambient humidity, temperature and contact pressure. The non-blocking quality is industrially important since envelopes, stamps and sealing tapes are often stored under high ambient humidity and in packages or rolls where tension and resilient contact pressure between adjacent layers may cause activation of the adhesive.
The water-activatable, hot-melt adhesive should, by definition, also develop a good adherent bond between the coated substrate and another coated or uncoated surface after activating the coating with water. The bond from the activated coating must also be consistent in that the application of water (or saliva) will uniformly wet and uniformly activate the coating or coated areas. Too often, the prior art adhesives, particularly after long storage, do not rapidly and uniformly wet and activate. The moisture droplets merely bead up on the surface of the adhesive layer and don't activate it.
The above problems have been recognized in the art. The prior-art compositions directed to water-activatable hot-melt adhesives are primarily based on plasticized N-vinyl lactam copolymers, specifically vinyl pyrrolidone-vinyl acetate (VP/VA) copolymers. Most of the adhesive plasticizers have been based on polyethylene glycol (PEG's). Specifically U.S. Pat. Nos. 3,345,32 and 3,462,342 are based on such compositions. Both are capable of storage without activation at up to 50% relative humidity. However 50% relative humidity is a condition requiring special storage facilities. In addition, the patented compositions particularly U.S. Pat. No. 3,462,342 have short pot-life at melt application temperatures. The plasticizers in these compositions were based on liquid PEG's of molecular weights of about 1000. The U.S. Pat. No. 3,852,231 adhesive, while non-blocking, is extremely heat sensitive. It requires that the pot temperature be kept below 300.degree. F. Should the pot temperature rise above 300.degree. F. for any time, self-polymerization will be initiated, causing the viscosity of the melt to rise. Within a short time, often hours, the pot contents will gel and ultimately solidify. This short pot-life severely limits the usefulness of these compositions.
U.S. Pat. No. 3,888,811, while non-blocking to about 83% humidity, is only non-blocking at such humidity levels at low pressures, i.e. up to about 0.5 p.s.i.g. At higher pressures blocking has been noted. Such blocking under pressure interferes with storage of the coated materials prior to activation.
U.S. Pat. No. 3,852,231 teaches hot-melt, water-remoistenable adhesives also based upon the aforesaid VP/VA copolymers modified by the addition of PEG as a plasticizer and adhesion promoter and phthalate or benzoate ester plasticizers. To control the setting of the hot-melt upon application to the substrate and also to some degree to prevent blocking when the softening point of the polymer is reduced (by choice of copolymer ratios or reduction of the amounts of ester plasticizers), a mineral-type wax is added to the melt. These waxes, are naturally occurring esters of long chain acids such as montanic acid (C.sub.27-28). Also mentioned in this reference is the recognized repulsive flavor of the prior art adhesives and a recommendation is made therein that urea and its precursors be added to promote good taste.
Spot-coated envelope blanks are usually stored in rolls prior to cutting and assembly. The contact pressure in such rolls, as well as sealing tapes, is commonly up to about 1.0 p.s.i. Similarly stamp rolls and stacked sheets of stamps often are subject to considerable contact pressure. The water-activatable adhesive-coated assemblies should not block even at 87% relative humidity and about 1.0 p.s.i.
Another criterion to be met by such water-activatable adhesives is that the resultant dry coating should have an acceptable taste. While moistening by saliva may be decried for sanitary reasons, the fact remains that it is a most convenient method for spontaneously applying moisture, i.e. "licking envelopes and stamps".
Also criteria to be met are the hot-melt characteristics of the formulations. The adhesive should have good pot-life, at least 30-40 hours at usual application temperatures of about 350.degree. F. It must have low viscosity characteristics at these temperatures and the viscosity of the hot-melt should be substantially constant during the pot-life of the hot-melt.
In addition, the selected adhesive formulations must be economically viable, i.e. should contain minimal amounts of the expensive VP/VA copolymer and/or esoteric ingredients such as the expensively purified mineral waxes.